1. Field of the Invention
The present invention relates generally to a method of measuring hydrogen concentration in conductive materials. In particular, the present invention relates to a method of measuring hydrogen concentration in nuclear fuel rod claddings using eddy current techniques.
2. Description of the Related Art
The useful lifetime of nuclear fuel rods is limited, in part, by the structural integrity of the zirconium alloy cladding encasing the radioactive material used to power the reactor. One measure of the structural integrity of the zirconium alloy cladding is the hydrogen concentration in the cladding. Hydrogen concentration is important because hydrogen precipitates as a brittle second phase. An average concentration across the cladding wall on the order of 600 parts per million (ppm) leads to measurable hydrogen embrittlement. However, the hydrogen concentration across the wall of the cladding is not uniform. The concentration at the outer surface (OD) of the cladding can be more than ten times higher than at the inner surface (ID) of the cladding, forming an OD rim of brittle second phase. The thickness of this rim depends primarily upon the amount of absorbed hydrogen and the temperature and temperature gradient across the wall during operation. This embrittlement leaves the cladding material susceptible to cracking, potentially leading to release of fissionable material into the reactor coolant. Therefore, determining hydrogen concentration in nuclear fuel rods is of great interest to the nuclear power industry.
The current method for determining hydrogen concentration in nuclear fuel rods is a destructive method, involving cutting samples out of the rod. These samples must be shipped to a laboratory that is equipped to handle radioactive materials for analysis. The analysis is a slow and expensive analytical process, requiring skilled operators and complicated equipment. Consequently, the number of analytical samples processed is relatively small, compared to the total sample population. This leads to a poor statistical representation of the fuel rod population, which may result in premature discharge of fuel rods, or more importantly, may lead to the use of fuel rods past their safe operating lifetime. Therefore, while this method is accurate, the fact that it is destructive, requires off-site analysis, and is slow limits its usefulness.
In-situ methods of hydrogen measurement are available. However, these in-situ methods involve indirect measurements and assumptions which, if untrue, lead to large errors in hydrogen concentration. One such method measures a parameter different than hydrogen concentration, the zirconium oxide thickness, which is then correlated to hydrogen concentration using certain assumptions. Zirconium oxide is formed at the surface of fuel rods in the reaction between the zirconium cladding and the water used as a moderator in light water reactors. During operation of the reactor, the high temperatures at the surface of fuel rods cause the water to react with the surface. The hydrogen left after the oxygen from the water has reacted either absorbs into the cladding or remains in the water. Generally, a significant fraction of the available hydrogen is absorbed by the metal. The zirconium oxide-based analysis proceeds as follows.
The thickness of the zirconium oxide layer is determined using an eddy current probe pressed against the surface of the fuel rod. This technique uses a probe containing a metal coil of wire, excited with radio-frequency alternating current, to induce a current, called an eddy current, in the fuel rod cladding. The induced eddy current in turn induces a back EMF in the probe coil and changes the probe impedance. The induced eddy current and probe impedance are dependent upon how close the probe is to the cladding, the effective electrical conductivity of the cladding, the effective magnetic permeability of any material between the probe and the cladding, and, if the excitation frequency is low enough, the thickness of the cladding. The presence of zirconium oxide, which is a non-magnetic electrical insulator, causes the probe to be separated from the metal cladding surface. The greater the zirconium oxide thickness, the further the probe is from the metal surface and the smaller the induced eddy current. Normally the excitation frequency is high enough such that the induced eddy current does not penetrate to the ID of the fuel rod cladding. The probe impedance is calibrated by using standards of the cladding material with various known thicknesses of insulating material placed between the probe and the cladding surface. Since only the conducting properties of the insulating materials are of importance, the insulating layer used for calibration need not be an oxide. Mylar films, for instance, are commonly used. The calibration curve combined with the in-situ eddy current impedance when the probe is pressed against the fuel rod is assumed to be a measure of the zirconium oxide thickness.
This zirconium oxide thickness measurement, however, is subject to a number of sources of error. First, there is often an additional surface layer, known as crud, formed by agglomeration of coolant corrosion products on the fuel cladding surface. There are many sources of corrosion and wear products in a reactor system, including pumps, piping, and cladding. These products may be small particles of metal oxide from metal surface oxidation or from metal-to-metal contact. The crud is not typically discernable from the zirconium oxide through eddy current methods since it is also a poor conductor, so the assessed oxide thickness is skewed by an amount equal to the thickness of the crud. This results in an erroneously high zirconium oxide thickness measurement. Because hydrogen concentration is correlated directly with zirconium oxide thickness, this also results in an erroneously high estimate of hydrogen level. The overestimation of oxide thickness and hydrogen concentration both lead to a determination that the fuel rods are closer to the end of their useful life than is actually the case. This could potentially lead to premature discharge of fuel rods and wasted money and resources.
A second condition that may be a source of error, related to crud deposition, is when ferromagnetic material is deposited onto the cladding surface. Zirconium oxide is non-magnetic. Deposition of ferromagnetic material onto the surface of zirconium oxide leads to a distortion of the radio frequency energy applied through the probe and a change in the eddy current response, which results in an even greater overestimation of zirconium oxide layer thickness. This error is sometimes corrected using the two dimensional nature of the eddy current response. The correction assumes, however, that fuel rod wall thickness and crud permeability are uniform. Neither of these assumptions is typically true, which leads to errors in estimation of oxide thickness and in hydrogen content. Such assumptions may lead to errors in calibration of 15% or more, depending on the actual circumstances. U.S. Pat. No. 5,889,401 to Jourdain, et al. discloses a refinement to the above approach in which measurements are iteratively fit to a mathematical model of the probe and its interaction with the fuel rod and environment to estimate zirconium oxide and crud thickness. This method has been extended to include measurement of hydride in U.S. Pat. No. 6,541,964, also to Jourdain et al. However, theses methods still do not directly measure the parameter of interest, hydrogen concentration. The former patent is only useful for oxide and crud measurement, while the latter patent still depends on an indirect, iterative, mathematical model approach. In addition, U.S. Pat. No. 6,541,964 does not address a number of key complications, including fuel rod temperature, temperature gradients, and inhomogeneous hydrogen concentrations.
What is needed is a direct method of measuring hydrogen content in nuclear fuel rods. The measurement should preferably be made in-situ and should be independent of the temperature at which the measurement is performed.